2,070 research outputs found
The Lockman Hole Project: new constraints on the sub-mJy source counts from a wide-area 1.4 GHz mosaic
This paper is part of a series discussing the results obtained in the
framework of a wide international collaboration - the Lockman Hole Project -
aimed at improving the extensive multiband coverage available in the Lockman
Hole region, through novel deep, wide-area, multifrequency (60, 150, 350 MHz,
and 1.4 GHz) radio surveys. This multifrequency, multi-band information will be
exploited to get a comprehensive view of star formation and active galactic
nucleus activities in the high-redshift Universe from a radio perspective. In
this paper, we present novel 1.4 GHz mosaic observations obtained with the
Westerbork Synthesis Radio Telescope. With an area coverage of 6.6 deg2, this
is the largest survey reaching an rms noise of 11 uJy/beam. In this paper, we
present the source catalogue (~6000 sources with flux densities S>55 uJy
(5sigma), and we discuss the 1.4 GHz source counts derived from it. Our source
counts provide very robust statistics in the flux range 0.1<S<1 mJy, and are in
excellent agreement with other robust determinations obtained at lower and
higher flux densities. A clear excess is found with respect to the counts
predicted by the semi-empirical radio sky simulations developed in the
framework of the Square Kilometre Array Simulated Skies project. A preliminary
analysis of the identified (and classified) sources suggests this excess is to
be ascribed to star-forming galaxies, which seem to show a steeper evolution
than predicted.Comment: accepted for publication on MNRAS. New version that corrects latex
errors and contain the correct version of figure 1
Accidental exposure to politics on social media as online participation equalizer in Germany, Italy, and the United Kingdom
We assess whether and how accidental exposure to political information on social
media contributes to citizens\u2019 online political participation in comparative perspective.
Based on three online surveys of samples representative of German, Italian, and British
Internet users in the aftermath of the 2014 European Parliament elections, we find that
accidental exposure to political information on social media is positively and significantly
correlated with online participation in all three countries, particularly so in Germany
where overall levels of participation were lower. We also find that interest in politics
moderates this relationship so that the correlation is stronger among the less interested
than among the highly interested. These findings suggest that inadvertent encounters
with political content on social media are likely to reduce the gap in online engagement
between citizens with high and low interest in politics, potentially broadening the range
of voices that make themselves heard
Evolution of the far-infrared luminosity functions in the Spitzer Wide-area Infrared Extragalactic Legacy Survey
We present new observational determination of the evolution of the rest-frame
70 and 160 micron and total infrared (TIR) galaxy luminosity functions (LFs)
using 70 micron data from the Spitzer Wide-area Infrared Extragalactic Legacy
Survey (SWIRE). The LFs were constructed for sources with spectroscopic
redshifts only in the XMM-LSS and Lockman Hole fields from the SWIRE
photometric redshift catalogue. The 70 micron and TIR LFs were constructed in
the redshift range 0<z<1.2 and the 160 micron LF was constructed in the
redshift range 0<z<0.5 using a parametric Bayesian and the vmax methods. We
assume in our models, that the faint-end power-law index of the LF does not
evolve with redshifts. We find the the double power-law model is a better
representation of the IR LF than the more commonly used power-law and Gaussian
model. We model the evolution of the FIR LFs as a function of redshift where
where the characteristic luminosity, evolve as
\propto(1+z)^{\alpha_\textsc{l}}. The rest-frame 70 micron LF shows a strong
luminosity evolution out to z=1.2 with alpha_l=3.41^{+0.18}_{-0.25}. The
rest-frame 160 micron LF also showed rapid luminosity evolution with
alpha_l=5.53^{+0.28}_{-0.23} out to z=0.5. The rate of evolution in luminosity
is consistent with values estimated from previous studies using data from IRAS,
ISO and Spitzer. The TIR LF evolves in luminosity with
alpha_l=3.82^{+0.28}_{-0.16} which is in agreement with previous results from
Spitzer 24 micron which find strong luminosity evolution. By integrating the LF
we calculated the co-moving IR luminosity density out to z=1.2, which confirm
the rapid evolution in number density of LIRGs and ULIRGs which contribute
~68^{+10}_{-07} % to the co-moving star formation rate density at z=1.2. Our
results based on 70 micron data confirms that the bulk of the star formation at
z=1 takes place in dust obscured objects.Comment: 17 pages, 14 figure
A subarcsecond near-infrared view of massive galaxies at z > 1 with Gemini Multiconjugate Adaptive Optics
We present images taken using the Gemini South Adaptive Optics Imager (GSAOI)
with the Gemini Multiconjugate Adaptive Optics System (GeMS) in three 2
arcmin fields in the Spitzer Extragalactic Representative Volume Survey.
These GeMS/GSAOI observations are among the first resolution
data in the near-infrared spanning extragalactic fields exceeding
in size. We use these data to estimate galaxy sizes, obtaining
results similar to those from studies with the Hubble Space Telescope, though
we find a higher fraction of compact star forming galaxies at . To
disentangle the star-forming galaxies from active galactic nuclei (AGN), we use
multiwavelength data from surveys in the optical and infrared, including
far-infrared data from Herschel, as well as new radio continuum data from the
Australia Telescope Compact Array and Very Large Array. We identify
ultraluminous infrared galaxies (ULIRGs) at , which consist of a
combination of pure starburst galaxies and Active Galactic Nuclei
(AGN)/starburst composites. The ULIRGs show signs of recent merger activity,
such as highly disturbed morphologies and include a rare candidate triple AGN.
We find that AGN tend to reside in hosts with smaller scale sizes than purely
star-forming galaxies of similar infrared luminosity. Our observations
demonstrate the potential for MCAO to complement the deeper galaxy surveys to
be made with the James Webb Space Telescope.Comment: 20 pages, AJ, in pres
Clustering of galaxies at 3.6 microns in the Spitzer Wide-area Infrared Extragalactic legacy survey
We investigate the clustering of galaxies selected in the 3.6 micron band of
the Spitzer Wide-area Infrared Extragalactic (SWIRE) legacy survey. The angular
two-point correlation function is calculated for eleven samples with flux
limits of S_3.6 > 4-400 mujy, over an 8 square degree field. The angular
clustering strength is measured at >5-sigma significance at all flux limits,
with amplitudes of A=(0.49-29)\times10^{-3} at one degree, for a power-law
model, A\theta^{-0.8}. We estimate the redshift distributions of the samples
using phenomological models, simulations and photometric redshifts, and so
derive the spatial correlation lengths. We compare our results with the GalICS
(Galaxies In Cosmological Simulations) models of galaxy evolution and with
parameterized models of clustering evolution. The GalICS simulations are
consistent with our angular correlation functions, but fail to match the
spatial clustering inferred from the phenomological models or the photometric
redshifts. We find that the uncertainties in the redshift distributions of our
samples dominate the statistical errors in our estimates of the spatial
clustering. At low redshifts (median z<0.5) the comoving correlation length is
approximately constant, r_0=6.1\pm0.5h^{-1} Mpc, and then decreases with
increasing redshift to a value of 2.9\pm0.3h^{-1} Mpc for the faintest sample,
for which the median redshift is z=1. We suggest that this trend can be
attributed to a decrease in the average galaxy and halo mass in the fainter
flux-limited samples, corresponding to changes in the relative numbers of
early- and late-type galaxies. However, we cannot rule out strong evolution of
the correlation length over 0.5<z<1.Comment: 14 pages, 9 (colour) figures. Published in MNRA
Mid-infrared sources in the ELAIS Deep X-ray Survey
We present a crossâcorrelation of the European Large Area Infrared Space Observatory (ISO) survey (ELAIS) with the ELAIS Deep Xâray Survey of the N1 and N2 fields. There are seven Chandra point sources with matches in the ELAIS Final Analysis 15âÎŒm catalogue, out of a total of 28 extragalactic ISO sources present in the Chandra fields. Five of these are consistent with active galactic nuclei (AGN) giving an AGN fraction of âŒ19 per cent in the 15âÎŒm flux range 0.8â6âmJy. We have coâadded the hard Xâray fluxes of the individually undetected ISO sources and find a low significance detection consistent with star formation in the remaining population. We combine our point source crossâcorrelation fraction with the XMMâNewton observations of the Lockman Hole and Chandra observations of the Hubble Deep Field North to constrain source count models of the midâinfrared galaxy population. The low dustâenshrouded AGN fraction in ELAIS implied by the number of crossâidentifications between the ELAIS midâinfrared sample and the Chandra point sources is encouraging for the use of midâinfrared surveys to constrain the cosmic star formation history, provided there are not further large undetected populations of Comptonâthick AGN
HerMES: A Statistical Measurement of the Redshift Distribution of Herschel-SPIRE Sources Using the Cross-correlation Technique
The wide-area imaging surveys with the Herschel Space Observatory at submillimeter (sub-mm) wavelengths have now resulted in catalogs of the order of one-hundred-thousand dusty, starburst galaxies. These galaxies capture an important phase of galaxy formation and evolution, but, unfortunately, the redshift distribution of these galaxies, N(z), is still mostly uncertain due to limitations associated with counterpart identification at optical wavelengths and spectroscopic follow-up. We make a statistical estimate of N(z) using a clustering analysis of sub-mm galaxies detected at each of 250, 350 and 500 ÎŒm from the Herschel Multi-tiered Extragalactic Survey centered on the Boötes field. We cross-correlate Herschel galaxies against galaxy samples at optical and near-IR wavelengths from the Sloan Digital Sky Survey, the NOAO Deep Wide Field Survey, and the Spitzer Deep Wide Field Survey. We create optical and near-IR galaxy samples based on their photometric or spectroscopic redshift distributions and test the accuracy of those redshift distributions with similar galaxy samples defined with catalogs from the Cosmological Evolution Survey (COSMOS), which has superior spectroscopic coverage. We model the clustering auto- and cross-correlations of Herschel and optical/IR galaxy samples to estimate N(z) and clustering bias factors. The S_(350) > 20 mJy galaxies have a bias factor varying with redshift as b(z) = 1.0^(+1.0)_(â0.5)(1 + z)^1.2^(+0.3)_(â0.7). This bias and the redshift dependence is broadly in agreement with galaxies that occupy dark matter halos of mass in the range of 1012 to 10^(13) M_â. We find that galaxy selections in all three Spectral and Photometric Imaging Receiver (SPIRE) bands share a similar average redshift, with = 1.8 ± 0.2 for 250 ÎŒm selected samples, and = 1.9 ± 0.2 for both 350 and 500 ÎŒm samples, while their distributions behave differently. For 250 ÎŒm selected galaxies we find the a larger number of sources with z †1 when compared with the subsequent two SPIRE bands, with 350 and 500 ÎŒm selected SPIRE samples having peaks in N(z) at progressively higher redshifts. We compare our clustering-based N(z) results to sub-mm galaxy model predictions in the literature, and with an estimate of N(z) using a stacking analysis of COSMOS 24 ÎŒm detections
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